Ring trip circuits are of course well known in the telephone art and these circuits respond to an off-hook condition at a called line to terminate the ringing. Most generally these circuits use relays to provide the isolation of the off-hook detector from line conditions while providing sensitivity enough to respond to the DC path closure caused by a station on the line going off-hook. More recently with the advent of optical couplers, a number of approaches have been developed using these components to provide DC detection with the necessary line to CO isolation.
In such systems, one major problem is to maintain the isolation provided by an optical coupler while maintaining the DC sensitivity of the off-hook detector at sensitive levels. The circuit must sense the off-hook DC current under the line condition extremes which may be encountered in a system. The circuit receives AC ringing signals which it must ignore while acting to detect a change in line conditions due to a called station going off-hook to shut off the ringing signals.
In the prior art use of optical couplers, variable resistors are commonly used with multiple strapping points to allow the circuit to be set individually for each line condition under which the circuit is to operate.